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Feb. 23, 2011 -- New research lends support to the idea that exposure to a wide range of microbes explains why farm kids have lower asthma rates than city kids.

School-aged children in the studies who lived on farms were about 30% to 50% less likely to have asthma than non-farm children who lived nearby.

Farm-dwelling children were also exposed to more bacteria and fungi than the other children.

The studies, which appear in the Feb. 24 edition of the New England Journal of Medicine, suggest a role for the so-called hygiene hypothesis in the development of childhood asthma.

According to the hypothesis, exposure to bacteria and fungi from environmental sources like dirt and animal hair early in life protects against asthma and allergies by helping the immune system develop normally.

It is theorized that increasingly clean environments may at least partially explain why asthma rates have doubled in developed nations in just the last three decades.

Comparing Asthma Rates

To test the theory, German researchers compared asthma rates among Bavarian children living on farms with those of children living in the same rural districts who had little direct contact with farms.

In one study, the researchers conducted DNA analysis on dust samples taken from mattresses of the two groups of children. In another, the researchers analyzed settled dust samples taken from elsewhere in the children’s bedrooms.

The analysis confirmed that kids living on farms had lower asthma rates and were exposed to a wider range of bacteria and fungi than children who did not live on farms.

There was also evidence that specific types of microbial exposures found mainly on farms played a role in the protection, study researcher Markus J. Ege, MD, of the University Children’s Hospital Munich tells WebMD.

“The farm environment is somewhat special, so there may be something about the dirt on farms that is protective,” Ege says.

While the German studies offered early clues as to what these exposures are, the next generation of research should provide much more information about the specific bacteria and/or fungi that protect against asthma, says University of Wisconsin pediatric allergy and asthma specialist James E. Gern, MD.

Vaccine Potential

Recent advances in the field, including the development of high-speed DNA sequencing devices and chip-based probes, will allow researchers to sequence much larger amounts of genetic material faster.

“With these technologies, we will be able to find 100 times more bacteria than were found in these studies,” he says.

The hope is that these studies will identify the specific microbes that protect against asthma, and that this will lead to vaccines or other treatments to prevent children from developing it.

“In the past we have thought that asthma results from contact with something bad in the environment, like tobacco smoke or air pollution,” Gern says. “While that may be true, it appears that there are also environmental factors that protect against asthma. That is very exciting.”